A conventional elevation angle measuring device comprises: a rotary disc which is turned at a constant speed and has a slit at a predetermined position; a reference point photoelectric detector fixedly arranged in the rotation area of the rotary disc; and a measurement point photoelectric detector installed on a rotary arm which is coaxial with the rotary disc. The device detects the output signals which are provided when the slit of the rotary disc passes through the measurement point photoelectric detector which has been turned towards the measurement point and the reference point photoelectric detector which has been fixed, to obtain the aimed angle of elevation according to the following expression:
Aimed Angle of Elevation=360 (degrees) x (time required for the rotary disc to turn from the reference point to the measurement point)/(time required for the rotary disc to make one revolution).
In measuring an angle of elevation with the conventional device, it is impossible to correct for the inclination of the device itself. In order to measure an absolute angle of elevation with the vertical axis at the absolute zero degree, it is essential to hold the device completely horizontal; however, in practical measurement, it is substantially impossible to avoid causing a slight inclination with the device.